1. Field of the Invention
The present invention relates to a method for polishing a thin plate, for example, a semiconductor wafer, a wafer made of quartz, ceramic material or the like, and to a holding plate for holding the thin plate, which is used during polishing.
2. Description of Related Art
Conventionally, polishing for a thin plate is carried out by giving a relative motion between the thin plate to be polished and a polishing cloth, i.e., polishing pad, while holding the thin plate on a holding plate and pressing the thin plate against the polishing pad by loading the holding plate from a polishing head, with supplying a polishing slurry between the thin plate and the polishing pad.
As the holding plate for holding a thin plate, one made of glass or ceramic Is generally used. However, one made of glass is relatively easy to bend when it is loaded from the polishing head. Because of the effect of bending, the peripheral portion of the thin plate tends to be pressed more strongly against the polishing pad, so that the peripheral portion may be polished more than the central portion thereof. As a result, there is a problem that the flatness of the polished surface of the thin plate is inferior to that of a ceramic plate because of non-uniform polishing in the surface of the thin plate. Therefore, when a high flatness is required, a ceramic plate is generally used as the holding plate.
Although the ceramic holding plate is manufactured through steps of forming and sintering, it is difficult to obtain a ceramic holding plate with precise dimensions for front and back surfaces by only such steps. Generally, in order to obtain precise dimensions therefor, a mechanical processing is performed on the peripheral, front and back surfaces of the plate after the forming and sintering. As the mechanical processing, lapping using a bonded diamond grinding wheel is performed. During lapping, a predetermined abrasive slurry is supplied, and after the lapping, an acid cleaning using an acid aqueous solution containing hydrochloric acid or the like, and a pure water cleaning are carried out for washing the abrasive slurry away.
When a polishing has been carried out with a ceramic holding plate which was made in the above-described manner, it has been ascertained that the following problem often occurs.
That is, when a polishing is carried out to thin plates by using a ceramic holding plate, in batch processing, it has been ascertained that there is a high probability that scratches due to polishing and the like are formed in the polished surfaces of all thin plates and that scratches, dents or stains are formed on the back surfaces of the thin plates. Because the existence of such scratches, dents or the like lead to an extreme drop of the rate of acceptable products, an urgent countermeasure thereto is desired.